Since the commercialization of optical cable, various cable designs have become known. In these designs. two relatively distinct techniques have been suggested or used to prevent or restrict migration of moisture along cable passageways. In one of these techniques, water repellant materials are included in the cable structures. These water repellant materials normally include hydrophobic greases or gels which are caused to fill cable passageways containing the fibers. There are problems associated with the use of greases or gels. For instance, such materials are difficult and costly to apply into and fill cable passageways, the filling operation necessarily taking place as parts of the cable which define the passageways are being formed. In more practical terms, tubes are made for enclosing the fibers, the tubes being extruded around the fibers as the fibers are guided through an extruder head together with the grease or gel which is applied into the tube under pressure. Grease or gel also makes it difficult and unpleasant to handle the fibers during installation or repair of a cable, and at low temperatures (e.g. below 0.degree. C.) change in viscosity of the grease or gel surrounding and contacting fibers may increase signal attenuation in the fibers. A further problem is that because greases or gels may be incompatible with economically desirable plastics which could normally be extruded as tubes for containing the fibers, more expensively engineered polymers may be required for the tubes.
In the other technique for preventing or restricting migration of moisture along the cable passageways, it has been suggested that the passageways should purposely remain unobstructed and pressurized gas (i.e. air) is pumped into the passageways to maintain a moisture-free environment. Such a structure has been described in U.K. Patent Application 2169098A in which pressurized air is caused to flow along grooves formed in a central plastics member of the cable so that the pressurized air can reach into spaces between tubes which surround the plastics member, each of the tubes containing optical fibers. With such an arrangement air flow detectors would be provided to sense a change in air flow rate, upon an escape of air through a damaged region in the outer elements of the cable, so as to trigger an alarm thereby signalling a need for cable repair.
While this latter technique thus provides a means for detecting cable damage, it does not provide a method for containing or preventing worsening of the damage. Hence, before repair can commence after the alarm signal, water may have entered the cable and have flowed freely in both directions from the originally damaged region. This flow does, of course, increase the length of damage along the cable and necessarily increases the cost of repair since the section of cable containing water will need to be replaced.